(a) Field of the Invention
This invention relates to web-feed control apparatus for synchronising the feed of a plurality of webs with respect to each other to facilitate subsequent simultaneous processing of the webs.
(b) Description of the Prior Art
In processing material webs it is often required to carry out some operation on the web itself with reference to a mark or print applied to the web. When processing webs of packaging material (plastic materials, paper and the like) is concerned, it is also often necessary to cut and/or weld and/or overprint the web with reference to a pre-established and repeated length and in a predetermined relationship to a reference pre-printed on each length. In practice it has proved impossible to carry out these operations based on measurements mechanically derived from the moving web insofar as slippage has been found to occur between traction members and the web and the elasticity of the web itself gives rise to uncertainty as to the positioning of printed markings.
It has therefore become the practice to use photocells which are arranged to sense marks provided along the web and thereby enable control of the position of the web to be corrected to its theoretical one (that is, its position if slippage and stretching were absent).
However, when it is desired to process two or more preprinted webs simultaneously on one machine the spread of deviations of the individual webs from their theoretical positions as determined by the set rate of web advance creates a considerable problem in simultaneously correctly positioning all the webs for processing by the machine.
To meet this problem the machine can be provided with independent web advance systems for each web. These advance systems detect by means of photocells the positional error of each web with respect to its theoretical position and then seek to adjust the web position to the theoretical one. Such a solution which has been used for a long time, is however complicated, expensive and liable to wear.